1. Field of the Invention
The present invention relates to a nonvolatile memory device provided with a metal-oxide-nitride-oxide semiconductor (MONOS) type etc. memory transistor including a nitride film in a plurality of insulation films between a semiconductor region in which a channel is formed and a gate electrode, and injecting hot carriers (high energy charges) into charge traps of a local area in the vicinity of a boundary plane between the nitride film and other insulation film and in a bulk layer of the nitride film to store data, and a method of producing the same.
2. Description of the Related Art
Paying attention to that hot carriers can be injected into a part of dispersed traps by a so-called channel hot electron (CHE) injection method, there is known a technology in which binary data is respectively written in a source side and a drain side of a nitride film to allow a storage of two bits of data per a memory cell (for example, referred to “Extended Abstract of the 1999 International Conference on Solid State Devices and Materials, Tokyo, 1999, pp. 522-523”).
In a description of the above document, a direction of applying a voltage between the source and the drain is changed, the two bits of data is written by the CHE injection, and a predetermined voltage is applied between the source and the drain in the reverse direction to a writing operation in a read operation, namely, a so-called “reverse read” method, which allows a read operation of the two bits of data accurately even if a writing time is short and an amount of retention charge is few. Also, an erase of data is performed by an injection of hot holes.
This technology can increase a speed for the writing time and reduce cost.
On the other hand, a nonvolatile memory injecting charges into a memory transistor to store data may be required to secure a difference of a threshold voltage (window width) in which at least a bit of data is able to discriminate after a retention of long duration at high temperature, so an improvement of the charge retention characteristics is important. In order to improve the charge retention characteristics, it may be required that a difference of initial threshold voltages is made large and that the threshold voltages hardly change in the retention of long duration at high temperature.
And additionally, it may be required that a speed for the writing and the erasing of data is made increase, operational voltages are made lower, and the feature thereof hardly change even if a rewriting of data is performed repeatedly (improvement of endurance characteristic).